Project Background: Prostate cancer is the leading male cancer. One in three men with prostate cancer is chemically castrated at some point with long-acting injectable drugs (i.e., androgen deprivation therapy or ADT). This impacts the well-being of thousands of men annually. Although some patients benefit in terms of survival and symptom improvement, chemical castration with ADT is also commonly performed when there are little to no health benefits to patients raising questions of low value care. A growing awareness of castration harms (e.g., heart attack, osteoporosis, loss of sexual function) creates patient safety concerns. Despite this, ADT use in low value cases, such as for localized prostate cancer treatment persists. Ineffective and harmful practices such as chemical castration of prostate cancer patients with ADT outside of the evidence base are ideal targets for de-implementation. De-implementation, or stopping low value practices, has the potential to improve patient outcomes and decrease healthcare costs. However, provider preferences regarding de-implementation are not well understood, and possible de-implementation interventions range from blunt formulary restriction policies to shared decision-making. Both intervention strategies need tailoring based on provider input for acceptability and feasibility in clinical practice, including piloting prior to trialing. As many medical practices lack evidence and cause harm, robust, behavioral theory-based methods for incorporating provider preferences into deimplementation strategy development will advance both implementation research and practice. Project Objectives: This study will use a theory-based, mixed methods approach to identify, tailor and pilot two different de-implementation strategies that vary widely in delivery, impact, and expected results for reducing low value ADT use, in preparation for a randomized comparative effectiveness trial. Project Methods: This innovative mixed-methods research program has three aims. Aim 1: To assess preferences and barriers for de-implementation of chemical castration in prostate cancer. Guided by the Theoretical Domains Framework, urologists from facilities with the highest castration rates across an integrated delivery system will be interviewed to identify key preferences and de-implementation barriers for reducing castration as prostate cancer treatment. This qualitative work will inform Aim 2 while gathering rich information for two proposed pilot intervention strategies. Aim 2: To use a discrete choice experiment, a novel barrier prioritization approach, for de-implementation strategy tailoring. A national survey of urologists will prioritize key barriers identified in Aim 1 for stopping castration as localized prostate cancer treatment using a discrete choice experiment design. These quantitative results will identify the most important barriers to be addressed through tailoring of two pilot de-implementation strategies in preparation for Aim 3 piloting. Aim 3: To pilot two tailored de-implementation strategies to reduce castration as localized prostate cancer treatment. Building on findings from Aims 1 and 2, two de-implementation strategies will be piloted. One strategy will focus on formulary restriction at the organizational level and the other on physician/patient decision-making. Outcomes will include acceptability, feasibility, and scalability in preparation for an effectiveness trial comparing these two widely varying de-implementation strategies. This innovative approach to de-implementation strategy development will transform how and why castration is performed for localized prostate cancer through combining provider preferences and strategy tailoring. This work will advance de-implementation science for low value cancer care and foster participation in a subsequent de- implementation evaluation trial by addressing preferences and concerns through pilot tailoring.